ADK & Custom MCP Server: Fixing Connection Closed Errors

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Troubleshooting 'Connection Closed' Errors with ADK and Custom MCP Server

Hey guys, if you're wrestling with the 'Connection closed' error when trying to connect your Agent Development Kit (ADK) to a custom MCP server, you're not alone! This can be a real head-scratcher, but let's break it down and get your ADK talking to your server. We'll walk through the common culprits and how to fix them.

Understanding the Problem: 'Connection Closed'

First off, let's get a handle on what this error actually means. The 'Connection closed' error in this context signals that the ADK is unable to establish or maintain a connection with your MCP server. Think of it like a phone call that gets cut off. The server is either not answering, hangs up immediately, or something in between is messing up the communication. This can happen for a bunch of reasons, like the server not running, the ADK misconfiguring its connection, or issues in the communication protocols.

Common Causes and Solutions

  1. Server Not Running or Improperly Configured:

    • Issue: The most obvious one! If your MCP server isn't up and running when the ADK tries to connect, you'll get this error. Also, it could be running, but not configured to accept connections. The server script might have errors that prevent it from starting correctly.
    • Solution:
      • Double-check: Make absolutely sure your MCP server is running. From the image you provided, it looks like your server is running, which is a good start! But still, ensure it's not crashing immediately after startup. Monitor the server's output in the console or log files for any error messages.
      • Verify Configuration: Make sure your server is listening on the correct port (if you're using one). In the provided code, it uses stdio, which means it communicates through standard input and output, so port configuration is not directly relevant. However, it's crucial that the ADK knows how to connect using stdio (more on that later).
      • Inspect Server Logs: Check the server's logs for any error messages. These can give you hints about why it's failing. Look for things like import errors, issues with tool loading, or problems with the MCP protocol implementation.
      • Important: In your provided server code, pay close attention to the asyncio.run(run_mcp_stdio_server()) line. If an exception occurs within run_mcp_stdio_server, your server might crash silently. Ensure the try...except block correctly catches and logs any potential errors.

  2. Incorrect ADK Configuration:

    • Issue: The ADK needs to know how and where to find your MCP server. If the ADK's configuration is wrong, it won't be able to establish a connection.
    • Solution:
      • PATH_TO_YOUR_MCP_SERVER_SCRIPT: This is the most likely culprit based on your description. Make sure the PATH_TO_YOUR_MCP_SERVER_SCRIPT variable in your ADK code (agent.py) is set to the absolute path of your my_adk_mcp_server.py script. The error message in the ADK output (the second image you provided) shows this is the source of the problem. Without the correct path, the ADK won't be able to find and execute your server.
      • Absolute Paths vs. Relative Paths: An absolute path starts from the root directory (e.g., /Users/yourusername/projects/mymcp/my_adk_mcp_server.py). A relative path is relative to the current working directory (e.g., mymcp/my_adk_mcp_server.py). Using the wrong kind of path is a very common mistake. In this case, you must use an absolute path.
      • Check command and args: In the ADK code, verify that the command in StdioConnectionParams is correctly set to python3 (or whatever command you use to run your server script), and the args list correctly contains the path to the server script.
      • Permissions: Although less likely, ensure the user running the ADK has the necessary permissions to execute the server script.

  3. Dependencies and Environment:

    • Issue: Missing dependencies or environment issues can cause the server or the ADK to fail to start properly.
    • Solution:
      • Install Dependencies: You mentioned installing dependencies, which is good. Ensure you have installed the necessary packages in both the mymcp and eight directories (as specified in your reproduction steps). Specifically, make sure you've installed the required packages in a Python environment. Use pip install -r requirements.txt inside each directory where a requirements.txt file exists.
      • Virtual Environments: Consider using virtual environments (e.g., using venv or conda) to manage dependencies. This helps prevent conflicts and keeps your project's dependencies isolated from the global Python environment. Always activate the virtual environment before running the server or ADK.
      • Python Version: Confirm that both the ADK and the server are running with compatible Python versions. Ideally, they should be using the same version or have versions known to work together.

  4. MCP Protocol Issues:

    • Issue: There might be an issue with how the MCP server and ADK are communicating. This could be related to message formatting, incorrect handling of requests, or issues with the MCP library itself.
    • Solution:
      • Inspect the Handshake: Add more logging to the MCP server to see what's happening during the handshake with the ADK. This can help you pinpoint where the communication is failing.
      • MCP Library Version: If you're using an MCP library, ensure you're using a compatible version with the ADK. Older or newer versions might introduce incompatibilities.
      • Error Handling in Server: Review the error handling in your server code, particularly within the call_tool function. Make sure that errors are being properly formatted and returned to the ADK in a way it can understand. Check if the JSON formatting is correct.
      • Debugging MCP Communication: Use a tool like tcpdump or Wireshark to inspect the actual data being exchanged between the ADK and the server. This can help you see the exact messages being sent and received.

Troubleshooting Steps - A Practical Guide

  1. Start with the Basics:

    • Server Status: Verify the MCP server is running without errors. Check the console output or any log files. Double-check that no error messages immediately appear on startup.
    • ADK Configuration: Carefully review your agent.py file. Pay special attention to the absolute path for PATH_TO_YOUR_MCP_SERVER_SCRIPT and make sure it points directly to your server script.

  2. Isolate the Problem:

    • Simple Test: Create a very simple MCP server (e.g., one that just returns a static message) to rule out problems with the ADK or the basic connection setup. If that works, the issue is likely within your more complex server.
    • Logging: Add extensive logging to both the ADK and the server. Log every step of the process: when connections are attempted, when messages are sent and received, and any errors that occur. This will provide crucial clues.
    • Simplify: Temporarily simplify your MCP server code to isolate the issue. Remove tools and complex logic to see if you can establish a basic connection. If it works, then add back functionality piece by piece.

  3. Examine the Details:

    • Error Messages: Read and understand all error messages from both the ADK and the server. They often contain valuable information about what went wrong.
    • Code Review: Thoroughly review the code for both the server and the ADK. Check for typos, logic errors, and potential issues related to the MCP protocol.
    • Reproducibility: Try to reproduce the issue on a different machine or in a different environment to rule out any system-specific problems.

Debugging Your Specific Code (Based on the Code Snippets)

Let's zoom in on your code and pinpoint potential issues.

  1. ADK Code (agent.py):

    • The Path: The error in the image you provided clearly shows that the ADK is failing to execute the server script. The primary fix is to ensure PATH_TO_YOUR_MCP_SERVER_SCRIPT is set to the correct absolute path. This is the most likely cause. Double-check that the path is correct.

  2. MCP Server Code (my_adk_mcp_server.py):

    • Server Startup: Make sure the server starts without errors. Check the console output when you run python3 my_adk_mcp_server.py. The screenshot you provided suggests it's running, but always double-check, and monitor any messages for issues on startup.
    • Error Handling: Review the error handling within the call_tool handler. Ensure that any exceptions are gracefully caught and that meaningful error messages are returned to the ADK in a JSON format that it can understand.
    • Debugging MCP Communication (stdio): Since you're using stdio, the communication is done via standard input/output. Check your server's code to see if it's correctly handling the input and output streams.

Final Thoughts

Debugging connection issues can be tricky, but don't get discouraged! Start with the fundamentals, isolate the problem with logging and simplification, and meticulously review your code. By systematically working through these steps, you'll be able to identify the root cause of the 'Connection closed' error and get your ADK connected to your custom MCP server.

Good luck, and happy coding!